The controller unit is the brains of the entire UV system. This is the part of the unit that controls the electrical output of the lamp, and powers it to produce UV light. Some controllers are very simple, and are a simple cap that fits over the end of the lamp and a plug. Other controllers, depending on the type of UV system, are more complicated, and have lamp-change timers, low-UV alarms, or trouble indicator lights to show when the system is not performing as it should